The present invention generally concerns a marking fluid containing hydroxypyrene trisulfonic acid as a fluorescence dyestuff or daylight fluorescing dye, in an aqueous alkaline medium. Hydroxypyrene trisulfonic acid is also referred to as pyranine or C.I. solvent green 7 No 59040, and that usage may be adopted herein.
A marking fluid of that kind is described in German patent specification No 2 315 680 and it can be used for clearly emphasising written, typed or printed tests or the like by being applied thereover, or to provide visual contrast for text from other pieces of text, thereby providing a marking or highlighting effect. The marking fluid described in the above-mentioned German patent has the advantage over other marking fluids that the marking or highlighting effect which is produced thereby on a document is highly color-intensive due to the daylight fluorescing dye character of the pyranine used and the coloration effect occurs spontaneously after application.
However certain disadvantages occur in some uses of the above-mentioned known marking fluids. If the pyranine-bearing fluid is applied for example to the copies of certain kinds of sets of copy paper which are referred to as `carbon paper-free` or `self-carboning`, it can be observed that the script or text to be marked is caused to fade or even completely erased while if the marking fluid is applied to heat-sensitive paper, for example what is known as `fax paper`, that can result in a darkening effect in regard to the marking area. Undesirable effects of that kind which are linked to the text to be emphasised or highlighted with the marking fluid becoming illegible may also occur in relation to papers with other coatings thereon.
In an endeavor to improve the fastness in relation to light of fluorescence dyestuffs acting as daylight fluorescing dyes, including those which contain pyranine, it is proposed in German patent specification No. 2 441 823 that reducing means should be added to the dyestuffs or the marking fluids containing same. Admittedly that may improve the fastness to light to a certain extent but the above-mentioned disadvantages involving fading or erasure of the text and a blackening or darking effect in regard to the marking area are not only not eliminated but under some circumstances and depending on the respective type of paper involved may even be worsened.